In U.S. Pat. No. 5,159,290 issued Oct. 27, 1992 in the name of Paul L. Bartley and Robert E. Myer for "Composite Wave Closure Means For Microwave Containing Regions", (the "Bartley-Myer patent"), and assigned to the assignee hereof and incorporated herein by reference and made a part hereof, there is disclosed microwave amplifying apparatus utilizing a cavity closure structure which encloses top and bottom circular microwave cavities. The upper side of the top cavity and the lower side of the bottom cavity are bounded by upper and lower rigid circular metal plates which are fastened together to be in axially spaced coaxial relation. The lower side of the top cavity and the upper side of the bottom cavity are, however, bounded over some or all of the area of the cavities by upper and lower sheet metal dishes which are axially spaced from each other and which, because made of sheet metal, are subject to flexure and lack the dimensional stability of the rigid plates in the presence of temperature variations, vibrations, and so on. While the flexibility of such sheet metal has not created an insuperable problem in the fabrication and use of the mentioned cavity closure structure according to the teaching of the Bartley-Myer patent, nonetheless, it has at times been difficult to attain the stability in the dimensioning of these cavities within the tolerances desired to be met.